In a spark-ignition internal combustion engine, the combustion mixture is ignited in the combustion chamber by a spark, usually generated by a spark plug. The exact timing of the spark is controlled by an ignition timing system. There are generally two types of ignition timing system.
The first type of ignition timing system is a contact-breaker system in which timing pulses are generated by a set of contact-breakers, also known as points. The contact-breakers are made to open and close in time with the rotation of the engine which causes a voltage to be periodically applied across the low voltage terminals of a transformer, known as a coil. In a four-stroke four-cylinder engine, four timing pulses of dc voltage per engine cycle are supplied across the coil (two pulses per crankshaft revolution). The inductive effect in the coil of the low voltage pulses causes high-voltage ignition pulses to be generated at the high-voltage, or HT, output of the coil, and these ignition pulses are fed to each spark plug in turn via a distributor. In this system, each low voltage timing pulse generated by the contact-breakers corresponds directly to a high-voltage pulse received by the spark plugs.
The second type of ignition timing system is a breakerless or electronic system. Timing pulses are generated in dependence on the rotation of the engine, normally either by means of an optical system including a perforated rotating disc and an optical sensor, or by means of magnets mounted in, for example, the crankshaft or camshaft of the engine passing a magnetic sensor. A fixed number of pulses is generated for each revolution of the engine, which is normally an integral multiple of the number of cylinders of the engine, such as 4, 8 or 12 pulses per revolution for a four cylinder engine. Therefore, the frequency and width of the pulses are dependent on the speed of rotation of the engine. The pulses generated do not usually correspond to the positions required for the high-voltage pulses to generate the spark, but are instead fed to an electronic ignition unit or engine management system which then generates the correct ignition pulses to be supplied to the coil or other high-voltage generating means.
Such ignition systems as described above offer no additional security should a thief gain access to the inside of the vehicle. For the thief to be able to drive the vehicle away, all that is necessary is for the thief to bypass the ignition lock to enable power to be supplied to the engine and starter motor. The engine can then be started and the vehicle driven away.
Many devices have been developed to increase vehicle security, such as additional locks, steering wheel, gear lever or handbrake clamps, alarms and immobilisers, however such devices are often employed to deter the opportunist thief and can be removed or bypassed by professional or determined thieves.
Engine immobilisers have been developed which are often coupled with a vehicle alarm system. If the alarm is triggered, the engine is immobilised and cannot be started. However, such systems often simply prevent the timing pulses from being supplied to the coil. If the thief resets the alarm, or prevents it from being triggered initially, the vehicle may become drivable again. Even if the alarm is triggered and the engine immobilised, a professional or determined thief can gain access to the immobiliser, usually in the engine compartment, and simply bypass the immobiliser which would then allow the timing pulses to be supplied to the coil and the ignition system to operate normally.
Other types of immobiliser are armed and disarmed by the use of an electronic password, which may be contained in an electronic key for example. A disadvantage with such systems, in addition to the ability of the immobiliser to be bypassed, is that an electronic key device belonging to the immobiliser of one vehicle could be used to disarm the immobiliser of another vehicle. This could occur if the password is the same for both immobilisers, which is a possibility as the number of combinations for the code are often limited in order to reduce the complexity of the electronic key device, or if the electronic password is altered to match that of the other vehicle.
An object of the invention is therefore to provide an engine security system such that the engine cannot be started when the device is bypassed.